This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
A common approach to making closely spaced metal nanoparticles is through advanced lithographic methods. Electron beam lithography is frequently used to make nanostructures on the scale of a few 10 s of nanometers in size and with similar spacing between nanostructures. However, prior efforts in this regard suffer from the inability to reliably produce features on this length scale with high precision. Defects in shape, size, and/or spacing generally arise when producing nanostructures on the scale of 10 nm in length using lithographic techniques. Currently, lithography techniques cannot reliably fabricate nanostructures with a spacing of 10 nm or less. Furthermore, lithography yields static structures and does not allow for optimization of optical and electronic coupling between the nanoparticles, as real-time control over the distance between nanoparticles is not achievable.
As a result of these limitations, attempts have been made to use rough mechanical means to essentially push two particles together. [J. Merlein et al., Nature Photonics 2, 230(2008)] Electron beam lithography has been used to create two nanoparticles with an initial spacing of about 85 nm. Subsequently, an atomic force microscope tip is used to push one of the particles closer to the other. This is a crude method to move nanoparticles closer, and suffers from deficiencies such as damaging the nanoparticles, the substantial time needed to accomplish movement, and a complete lack of reversibility in the degree of coupling between the nanoparticles. Attempts at applying MEMS to move two “swords” of silicon coated Au structures to near contact have also been made [IEEE Transducers 2009 conference (Jun. 21-25, 2009)]. Here, the silicon swords had micron lengths and included coarse position control.
Achieving nanoparticle separation on the scale of less than 10 nm is generally beyond the reliable capability of modern nanofabrication tools. In addition to the nanoparticle spacing issue, the ability to modulate this distance in a reversible manner would be of value for a wide range of next generation nanoscale devices. For example, controlled coupling of nanoparticles could be used as a switching framework to control optical and electrical energy flow in nanoscale devices. Another example that could benefit from this technology would be a sensor where strongly interacting fields of the involved nanoparticles produce shifts in the absorption and scattering resonances of the particles. Another benefit of close nanoparticle coupling would be the gain in the sensitivity of spectroscopies, including surface enhanced Raman spectroscopy (SERS), that result from large field confinement and enhancement effects in the gaps between nanoparticles. For example, by positioning molecules within this small region, molecular spectroscopy of a single molecule can be achievable. Precise control of the distance between nanoparticles is also needed to manage the efficiency of bow-tie nanoantennas. Nanoantenna structures are best known for providing a mechanism for focusing light into the nanoscale gap between conductors. It is expected that these structures could ultimately provide optical readout for on chip nanophotonic logic or light routing devices.